The present invention relates to tube-bending mandrels, and more particularly concerns such a mandrel having ease of assembly and increased restraint against relative displacement of the assembled parts.
Tube-bending mandrels are widely employed to support the inside of a tube as it is being bent. The mandrel is inserted into the section of a tube that is to be bent, and as bending takes place, the mandrel, which is flexible, bends with the tube but supports the inside of the tube and prevents collapse or undue distortion. After completion of the bend, the mandrel is withdrawn, an operation that requires a very large force because the mandrel must be withdrawn through the bent portion of the tube. The bent portion of the tube has been somewhat distorted during the bending, partly by being forced to conform to the shape of the flexible mandrel sections, and therefore some of this distortion is necessarily removed by the withdrawing of the mandrel, thus increasing the resistance to such withdrawal.
The tube-bending mandrel commonly includes a straight section ending in a mandrel shank to which is connected a shank link. To the shank link are connected, in end-to-end relation, a number of articulated links generally of a ball-and-socket configuration. Mounted on these articulated links are ball segments which collectively form the outer surface of the flexible mandrel and provide the contact with the inner surface of the tube being bent. The flexible mandrel section, of course, must be readily assembled and disassembled. Partly because of the large forces involved, the parts are subject to relatively rapid wear. Thus, the mandrel may be disassembled frequently for repair and replacement of parts. For this reason, primary criteria dictating the design of the flexible mandrel are ease of assembly and disassembly. The desired ease of replacement of parts and the large forces experienced by the assembled device have resulted in many different configurations and constructions, developed for enabling assembly and disassembly of the mandrel linkage.
Longitudinally-split mandrel links have many features that facilitate assembly and disassembly. However, such arrangements suffer from disadvantages that derive from the longitudinally-split configuration. The U.S. Pat. No. 3,190,106, to Spates, illustrates a tube-bending mandrel employing longitudinally-split links, and also illustrates the disadvantages of such construction, which have led subsequent workers in the field to avoid the longitudinally-split link. The two halves of the center link, which abut along a longitudinal parting plane, are restrained against relative longitudinal sliding by a key and keyway. However, as previously mentioned, the mandrel is subjected to very large forces. Moreover, such forces are exerted on the device in many different directions, and seldom symmetrically. As the mandrel is withdrawn, the outer ball segments are frictionally restrained by the bent and somewhat disfigured surfaces of the bent tube section. Therefore, unless the key and keyways are precisely fit, mated and positioned, the two link haves are subject to longitudinal displacement, which may transfer relatively larger proportions of the applied forces to one of the link halves and lesser forces to the other. This results in more rapid wear, or even failure of the parts.
Another disadvantage of prior mandrel constructions is the large number of different parts that are employed. In order to assemble the ball-and-socket joints, each of the links must be made in two or more parts, fitting together in the manner of a three-dimensional jigsaw puzzle. In some constructions more than three sections are employed for each link. Thus, for repairability and replacement, one must maintain relatively large quantities and stocks of a number of different parts. This is expensive, time consuming and tends to greatly increase the chances of error in assembly if wrong parts are selected during the assembly process.
Accordingly, it is an object of the present invention to provide a strong, easily assembled and disassembled tube-bending mandrel that avoids, or at least greatly alleviates, the above-mentioned problems.